1. Field of the Invention
The present invention relates to the field of memory devices and particularly to external memories configured with a microcontroller.
2. Related Application
This application is related to co-pending application Ser. No. 07/707,241, filed May 22, 1991 (now U.S. Pat. No. 5,077,738) which was a continuation of Ser. No. 07/292,366, filed Dec. 30, 1988, now abandoned and entitled "TEST MODE ENABLE SCHEME FOR MEMORY".
3. Prior Art
In the area of semiconductor memories, the design and manufacture of erasable programmable read only memories (EPROM) are well-known in the prior art. These EPROM devices are formed on a semiconductor chip and typically configured as standard size memory, such as 32K or 64K. Typically these memory chips are disposed in a standard package. Semiconductor memory devices, such as EPROMs, are coupled to operate in conjunction with other semiconductor devices. In most instances, these EPROMs are coupled to processors which control the transfer of data between it and the memory device. In a basic configuration, a certain memory location of the EPROM is accessed by the processor when the processor generates address signals on address lines coupled to the memory. Data is written or programmed into the memory or read from the memory depending on the control signals provided by the processor. The data transfer is achieved by placing the appropriate information on the data bus coupled to the memory. Unless the EPROM is part of a larger structure, such as a programmable logic array device, the EPROM does not include processing circuitry other than the circuitry needed for addressing and providing data transfer.
One group of processors used to operate with EPROMs are known as microcontrollers. A microcontroller is a specialized processor used to meet specific, including custom, application needs. These controllers are self contained units and typically can include a processor, logic circuits, timing and control circuits, buffers, latches, and on-chip memory or memories. In most instances, the specific application software is embedded in the controller chip. These controllers also include input/output (I/O) ports for the purpose of transferring information to and from the controller.
However, whenever external memory devices, such as an EPROM described above, are coupled to a given controller it is coupled to one or more of the ports of the controller. That is, if a given microcontroller requires an off-chip memory for a given function of the controller, then the off-chip memory is coupled to one or more of the ports of the controller, wherein these ports are lost for I/O use. Without the use of additional off-chip circuitry the coupling of the external memory to a microcontroller places severe limitation on its I/O capability, because the external memory monopolizes one or more of the microcontroller ports.
It is appreciated that what is required is a scheme for coupling external memory to a microcontroller without reducing the number of I/O ports of the controller.